Why is this world denomination come in the 1000s? Inflation much?
Because too many isekai authors consider their readers to be idiots who can't understand any other currency but the Japanese yen. Yen is one of the currencies that hasn't got a subunit at all (cent and penny being rather famous examples). For practical purposes, in order to function more or less the same in everyday life, yen is (very) roughly 100 times cheaper than dollar or euro.
These isekai authors won't spend a single millisecond to think of the consequences of their modern yen-equivalent currency and it will never be addressed in the work, typically. A person hands over a single coin, such as a silver coin, and says it's 1000 whatever currency. They could just say it's a silver coin, but instead they must say it's 1000 definitely-not-yen.
I haven't got anything against denomination names in fantasy, of course. For example silver crown, gold crown do actually sound more nice than simply saying silver coin, gold coin. It could be Country A has silver crown, whereas Country B has silver thaler. They might have different amounts of silver, so the crown might be bigger (has more silver) and thus more valuable than the thaler, and the characters would need to take this into account when dealing with money. However, isekai authors believe this is too much for the Japanese audience.